1. Field of the Invention
The present invention relates to a circuit for receiving a transmission signal and associated method, and more particularly, to a circuit for dynamically compensating a baseline wander of the transmission signal and associated method.
2. Description of the Prior Art
Please refer to FIG. 1, which is a schematic diagram of typical user terminals 10A and 10B connected to each other by a network transmission line 18. The user terminals 10A and 10B can be network servers, routers, clients, or other kinds of network terminals. The transmission line 18 is a network transmission line, such as an Unshielded Twisted Pair Category 5 (UTP-Cat.5) Ethernet cable. The user terminal 10A and the user terminal 10B utilize a signal circuit 12A and a signal circuit 12B respectively to transmit signals via the transmission line 18. The signal circuits 12A, 12B include transformers 16A, 16B and resistors R0a, R0b respectively that match the impedance of the transmission line 18. A transmitter 14A of the signal circuit 12A utilizes a differential transmission signal. That is, the transmission signal includes both positive and negative transmission signals transmitted by a pair of conductive lines.
The differential transmission signal is sent through two output terminals of the transmitter 14A to nodes P0A, P1A, and is then transformed onto the transmission line 18 by the transformer 16A. The differential transmission signal is received by the signal circuit 12B of the user terminal 10B after the differential transmission signal travels through two wires of the transmission line 18. The differential transmission signal is transformed by the transformer 16B and is sent to nodes P0B, P1B of the signal circuit 12B. The differential transmission signal is then sent to two differential input terminals of a receiver 14B, so that the user terminal 10B receives the signal from the user terminal 10A via the transmission line 18.
Several problems can occur during the signal transmission process. For instance, each signal circuit must match the electrical characteristic of the transmission line 18 in order for signal transmission to be effective. However, a high pass characteristic of the transformer in the signal circuit undesirably affects the signal level of the transmission signal.
The Ethernet network system suffers from this high pass problem. In Ethernet, an MLT-3 coded transmission signal is sent from the transmitter, transformer, to the transmission line. The coded transmission signal has a fixed baseline that is a longtime average of a level-changeable digital signal. The electrical characteristic of this signal is that the baseline of the signal relates to a low frequency (LF) component of the signal, which relates to the transmission data. When the coded transmission signal passes through the transformer and onto the transmission line, the direct current (DC) component of the LF is filtered out due to the high pass characteristic of the transformer. After the transmission signal passes through the transmission line to the signal circuit of another user terminal, the original baseline of the signal is degraded and baseline wander occurs.
For the purposes of explaining the baseline wandering phenomenon, please refer to FIG. 2 showing a prior art waveform-time diagram of the transmission signal during the signal transmission process in the network system shown in FIG. 1. The horizontal axis in FIG. 2 is time, and the vertical axis is signal amplitude. In FIG. 2, because the transmission signal is a differential signal, a waveform of a positive transmission signal shown is representative. At the user terminal 10A in FIG. 1, the transmission signal is generated as illustrated by a waveform 20. The waveform 20 is composed of three different waveform levels: a high level, a zero level, and a low level representing the different digital signals according to the MLT-3 code. A dotted line 20A and a dotted line 20B perfectly represent the overall signal profile of the waveform 20.
After the transmission signal passes through the transformer 16A, the transmission line 18 and the transformer 16B of the user terminal 10B, the LF component is filtered out due to the high pass characteristic of the transformer. The transmission line affects the original baseline and baseline wander occurs. A waveform 22 of the transmission signal affected by baseline wander is received at the node P0B of the user terminal 10B. The level of the waveform 22 has different deviations at different times, so that enveloping signal levels 22A and 22B of the waveform 22 appear to wander between a high level and low level. Therefore, the waveform 22 does not represent the original digital signal of the waveform 20 correctly because of the baseline wandering phenomenon.
Signal transmission errors caused by the baseline wandering phenomenon will now be briefly explained with reference to FIG. 1 and FIG. 2. A signal with a level higher than a fixed high reference level 24A is determined as a high level digital signal. A signal with a level lower than a fixed low reference level 24B is determined as a low level digital signal. The waveform 20 is originally generated by the transmitter 14A with respect to the high reference level 24A and low reference level 24B. The waveform 20 is then affected by the baseline wandering phenomenon during its transmission as previously described, and is finally received at the receiver 14B. The receiver 14B then interprets the waveform 22 with reference to the high reference level 24A and low reference level 24B and generates a received signal waveform 24.
The waveform 24 contains errors in time periods T1 and T2. In time period T1 the waveform 22 wanders low enough so that the original high signal in period T1 falls below the high reference level 24A. The original high signal during period T1 is interpreted as zero level as shown by waveform 24, incorrectly. A plurality of low level pulses error in a similar way during time period T2. The baseline wandering phenomenon thus causes the digital signal of the network transmission to not be received and decoded correctly.
In the prior art signal circuit, a fixed DC bias is added to the received transmission signal at the receiving terminal to compensate for baseline wandering. This solution is not adequate as the deviation of the baseline changes over time, and a fixed compensation cannot eliminate the signal deviation phenomenon entirely.